(1) Field of the Invention
The present invention relates to an image scanning apparatus and particularly an image scanning apparatus including an optical magnification varying mechanism capable of super-or oversize magnification.
(2) Description of the Prior Art
A conventional image scanning apparatus has a construction as illustrated in FIG. 6. In the illustrated apparatus, an original document (not shown) placed on a document table 101 consisting of a glass plate is illuminated by light emitted from a light source 102 and condensed by a reflecting mirror 103. The light reflected by the document travels via mirrors 104, 105 and 106 to a lens 107 and forms an image on an array of light receiving elements 108. The light source 102, reflecting mirror 103 and mirror 104 are movable as a unit to scan the document, and the mirrors 105 and 106 connected thereto by an unillustrated wire or the like are also movable together, whereby the optical path has a constant length during scanning operation. The apparatus for scanning the document and converting its image into electric data by the light receiving elements as above is generally called an image scanner.
In enlarging the document image with this type of image scanning apparatus which employs the photo-electric conversion, it is more advantageous in terms of reproduced image quality to provide an optical enlargement of the image rather than an electrical one. Thus the magnification is changed by varying the distances between the original document and the lens and between the lens and the light receiving elements. The document to lens distance is varied by actuating an unillustrated motor to move a movable support 110 carrying the lens 107 along a guide member 109 in directions following the optical path. The lens to right receiving element distance is varied by actuating a motor 113 to move, by means of a lead screw or the like, a slider 112 carrying the array of light receiving elements 108 along a guide 111 attached to the movable support 110.
In reproduction with the above construction, document to lens distance a.sub.1 mm, lens to light receiving element distance b.sub.1 mm, focal length of the lens fmm and magnification .beta. are in the following relationship: EQU a.sub.1 =-f (1-1/.beta.) EQU b.sub.a =-f (1-.beta.)
Assuming that the light receiving elements 108 have a pitch of 7 micrometers and the reproduction density is 400 dpi (dots per inch), then the magnification .beta. provided by this optical system will be -1/9.07. Where the focal length f is 50 mm, a.sub.1 will be 503.3 mm and b.sub.1 55.5 mm. By arranging the documents, lens and light receiving elements at these distances, the reproduction may have a 400 dpi resolving power.
On the other hand, providing for the reproduced image at a double size, the above magnification .beta. is doubled to be -2/9.07. Then document to lens distance a.sub.2 and lens to light receiving element distance b.sub.2 will be 277.8 mm and 61.0 mm, respectively.
It will be appreciated that, when the reproducing magnification is changed to the double size, the document to lens distance must be reduced by 222.5 mm and the lens to light receiving element distance increased by 5.5 mm.
If the reproducing magnification is further increased, the document to lens distance must be reduced even more and ultimately the lens 107 would have to be brought into contact with the mirror 106.
The focal length f of lens 107, on the other hand, needs to be increased to realize a reduced angle of view. The focal length f of about 50 mm as mentioned above is considered appropriate since an increased focal length would elongate the optical path and require the entire apparatus to be enlarged accordingly. Consequently, the lens 107 is movable within a range of about 200-300 mm only, which sets a magnification limit to double size or thereabouts. It is thus difficult for existing image scanning apparatus to realize reproduction of, for instance, film images magnified several times.